The synthesis of triglycerides in rabbits via the monoglyceride pathway has been shown to exhibit a market "surge" prior to birth. In addition the activty of the enzyme system has been shown to be increased in the upper third of the intestine when animals are fed a high triglyceride diet. The activity of the enzyme system has also been shown to be present in significant amounts in the lower small bowel following the ingestion of a high triglyceride diet for 7 days. The enzymatic activity in the normal animal is very low in this segment. These observations are consistent with the working hypothesis that an induction of the enzyme system can occur. The system may possibly be substrate induced. Using a specific antibody to rat liver cytchrome b5, ethanolamine plasmalogen (1-O-alk-1-enyl-2-acyl-sn-glycerophosphorylethanolamine) synthesis by pork spleen microsomes is shown to require cytochrome b5. Either NADH or NADPH could serve as a source of reducing equivalents. These results provide direct evidence for the involvement of cytochrome b5 in plasmalogen biosynthesis from the corresponding 1-O-alkyl ether and suggest a pathway analogous to the fatty acid desaturase system.